In processing color image signals obtained by a plurality of image sensing devices with different color filters, white balance adjustment is performed for the color image signals. In white balance adjustment, faithful color reproduction of a sensed image is implemented by determining the color temperature of a light source for the sensed image and adjusting the white balance such that the sensed image becomes achromatic.
White balance adjustment methods include a method of determining the color temperature of the light source using a colorimetric element, a method of searching for an achromatic portion in the sensed image and determining the color temperature of the portion, and the like. In some cases, these methods cannot accurately adjust the white balance due to erroneous detection of the color of the light source or the like.
One of the most accurate white balance adjustment methods is a method of image-sensing an achromatic object (to be referred to as white-paper image sensing hereinafter), calculating a control value such that the proportions of the color components of a sensed image signal are equal to each other, and performing white balance adjustment using the control value in photography after that (to be referred to as manual white balance (MWB) hereinafter).
It is known that the photoflash light color temperature of an electronic flash used in photography changes depending on photoflash light conditions such as a voltage across two terminals of a flashbulb, the amount of photoflash light, and the like. The photoflash light color temperature becomes high when a voltage to be applied to the electronic flash is high or the amount of photoflash light is small while it becomes low when the applied voltage is low or the amount of photoflash light is large.
FIG. 10 is a graph showing the relationship between the amount of photoflash light of an electronic flash and the photoflash light color temperature.
For example, assume that white-paper image sensing and MWB are performed when the amount of photoflash light of the electronic flash is “1/2”. In this case, a control value corresponding to a color temperature T2 is calculated, and accurate white balance adjustment is performed.
If a plurality of types of electronic flash devices can be used in an electronic still camera, the camera side performs white balance correction of photoflash light by notifying the camera side of the color temperature information of an electronic flash device to be used. Some electronic still cameras correct an output from a color temperature setting circuit in accordance with a time period from the start of photoflash to the stop (see Japanese Patent Laid-Open No. 02-85834).
However, if MWB is set for the amount of photoflash light of a certain electronic flash in the above-mentioned manner, and the amount of photoflash light of the electronic flash changes to “1/1” in subsequent electronic flash photography, the photoflash light color temperature becomes T1 in FIG. 10 which is a color temperature lower than T2. If white-paper image sensing data when the amount of photoflash light is “1/2” is used in this state, an image turns red, and accurate white balance adjustment cannot be performed. On the contrary, if the amount of photoflash light changes to “1/4” in next electronic flash photography, the photoflash light color temperature becomes T3 which is a color temperature higher than T2. If white-paper image sensing data when the amount of photoflash light is “1/2” is used in this state, an image turns blue, and accurate white balance adjustment cannot be performed. Therefore, to perform accurate white balance adjustment in photoflash light, white-paper image sensing needs to be performed for each amount of photoflash light of an electronic flash device.